The integrated circuit industry has generally accepted the fact that thin film transistors (TFTs) are necessary for use in some applications. A portion of the integrated circuit industry that requires TFTs for future family generations is a market involving memory products such as fast static random access memories (FSRAMs). Research conducted on TFTs has lead to FSRAM circuits that have: (1) less array current leakage and less parasitic stand-by current; (2) less occurrences of leakage-current-induced soft errors; (3) improved resistance to soft errors and increased cell capacitance; and (4) a higher logic "on" voltage.
Although over-gated or top-gated thin film transistors have been researched and in some cases designed into FSRAM load transistor applications, under-gated or bottom-gated transistors are more popular due to FSRAM bit cell area packing concerns. An over-gated transistor is a transistor that has a source, a drain, and a channel region that are formed underlying a gate region. An under-gated transistor is a transistor that has a source, a drain, and a channel region that are formed overlying a gate region.
The use of a conventional under-gated TFT has several disadvantages: (1) the source and drain implants are not aligned to the TFT gate; therefore, the TFT is subjected to etch and photolithographic variation as well as substantial device performance alteration due to alignment-related variation; (2) drain offsets are lithographically defined and, therefore, consistent current leakage limiting is not achieved; and (3) many TFTs are performance limited by short channel behavior, such as an increase in charge carrier punch through as a TFT gate length is decreased.